Ink jet printing is a non-impact printing process in which ink droplets are deposited on a substrate, such as paper or transparency film, in response to an electronic signal. Both dyes and pigments have been used as colorants in ink jet inks, with dyes generally offering superior colors as printed, and pigments generally providing superior light-fastness and durability of the printed images.
There are three primary attributes of ink jet inks: print quality, drying rate, and reliability. Print quality is primarily determined by optical density of the print, edge acuity, dot shape, and spray, if any. To a lesser degree, print quality is also determined by the printer mechanics such as resolution, drop volume, drop velocity, and the software controlling the print modes. But, it is the ink/paper interaction that has the dominant influence on print quality. Paper quality varies greatly depending on the manufacturing process. An ink, therefore, typically will interact differently with different commercial papers. It is desirable, however, that the ink produce high print quality independent of the print media.
Ink drying rate will strongly influence printer throughput. Ink jet printer throughput is relatively slow compared to printers using competing technologies, mainly due to "slow" drying rate of ink jet inks. Inks usually dry by two mechanisms: evaporation and penetration. Evaporation is determined by the vehicle vapor pressure, whereas penetration is determined by interfacial energy between the ink and the paper and the porosity of the paper.
Heating devices may be used to increase drying rate. Such devices, however, also accelerate solvent evaporation from the nozzles and may cause nozzle plugging. Heaters also add to the printer cost. Penetrating cosolvents and surfactants frequently are used to increase drying rate, but may degrade print quality by destabilizing the ink and/or increasing feathering.
Ink reliability issues include 1) decap or crusting time, which is the time a print head can stay dormant without failing to print; 2) drop volume consistency over pen life; 3) compatibility with the print head components; 4) long term storage stability and 5) robustness towards variable print parameters, such as the firing frequency and pulse width.
Methods to achieve these three primary ink attributes conflict with each other. For example, methods commonly used to increase drying rates, such as the addition of volatile cosolvents, either adversely affect print quality by causing increased feathering, or adversely affect reliability by causing the nozzle to be subject to crusting.
Accordingly, a need exists for improved inks that will produce high print quality on a wide range of papers typically used with ink jet printers, that are fast drying, and exhibit a high degree of reliability over extended storage times.